Chemical Bonding

1
Chapter 8
Chemical Bonding
2
Overview

• Lewis Symbols Octet
• Ionic Bonding
• Electron Configurations and Ions
• Ion Sizes
• Covalent Bonding
• Multiple Bonds
• Bond Polarity
• Electronegativity

3

• Lewis Structures
• Formal Charge
• Resonance Structures
• Exceptions to Lewis Rules
• Electron Deficient
• Expanded Valence
• Radicals
• Covalent Bond Strengths
• Bond Enthalpies
• Bond Length
• Oxidation Numbers

4
Lewis Symbols and Octet Rule

• Lewis symbols
• symbols showing valence electrons for an atom or
  ion
• for active metals representative elements
• number of valence electron group number
• for representative elements
• stability requires 8 electrons -- an octet
• like the nobel gases

5
Lewis Dot Symbols

C
N






1s22s2p63s1 1s22s22p2 1s22s22p3
O


1s22s22p4 1s22s22p5
6
Ionic Bonding

• Ionic Bond
• strong attractive, electrostatic force between
  cations and anions
• Ions form to achieve noble gas configuration an
  octet
• ion symbols shown as Lewis symbols
• generally forms between metals and non-metals
• overall, the formation of ionic bonds releases
  energy, exothermic

7
Atoms
Ions

Cl
-

Na

both have octets
electropositive metal
nonmetal with high electron affinity
Na(s) ½Cl2(g) NaCl(s) DHfo -
411 kJ/mol
8
Atoms
Ions
_
Cl
2

Mg
_
Cl
electropositive metal
all have octets
nonmetals with high electron affinity
9

• Energetics of Ionic Bond Formation
• formation of ionic compounds is exothermic
• electrostatic attraction E k Q1Q2
  lattice energy d
• Q1 charge of cation, Q2 charge of anion, d
  distance between cation and anion
• steps for formation
• change Na from solid to gas -- endothermic
• dissociate Cl2(g) -- endothermic
• remove electron from Na -- endothermic
• add electron to Cl -- exothermic
• combine ions, electrostatic attraction -- very
  exothermic
• sum of all above is exothermic

10
Practice Ex. 8.1

• Which would you expect to have the greatest
  lattice energy
• AgCl CuO CrN
• AgCl Q1 1 Q2 -1
• CuO Q1 2 Q2 -2
• CrN Q1 3 Q2 -3
• CrN because E k Q1Q2 is largest
  d

11
Problem

• If NaCl has a high lattice E and is very stable,
  would you expect NaCl2 to be even more stable?
• NaCl2 does not form -- even though the lattice
  energy would be higher, the formation of Na2
  would be so costly, so endothermic, that it would
  overwhelm the exothermic lattice energy

12
Formation of Ions

• Representative Elements
• last electron entered in an electron
  configuration is the first electron lost in ion
  formation

11Na 1s2 2s2 2p6 3s1 11Na 1s2 2s2 2p6
3s0

• Transition Metals
• the s electrons are always lost first, before any
  d electrons are lost

26Fe Ar 4s2 3d6 26Fe2 Ar 4s0 3d6
26Fe3 Ar 4s0 3d5
13
Sizes of Ions

• Cations Þ smaller than atoms from which they
  are derived
• removal of one or more electrons will
• increase Zeff felt by remaining electrons
• causes contraction of cation
• Anions Þ larger than atoms from which they are
  derived
• addition of one or more electrons will
• decrease Zeff felt by outer electrons
• causes some repulsion between electrons creating
  some expansion of size

14
Covalent Bonding

• A pair of electrons shared between two atoms
• Generally occurs between two non-metalsso that
  atoms can attain an octet
• Releases energy upon formation -- exothermic
• Two atoms can share
• one pair of electrons , single bond
• two pair of electrons, double bond
• three pair of electrons, triple bond

15
H
H


One Shared Pair of Electrons Single Bond
16
O
O




Two Shared Pair of Electrons Double
Bond
17


N
N




N
N
Three Shared Pair of Electrons Triple
Bond
18

• The more electrons, the shorter stronger the
  bond
• Bond strength
• single lt double lt triple
• Bond length
• single gt double gt triple

N - N N N N?N 1.47 Å 1.24Å 1.10Å
163 kJ 418 kJ 941 kJ
19

• Two types of covalent bonds
• non-polar
• electrons are shared equally
• atoms sharing electrons have equal attraction for
  them

• polar
• electrons are not shared equally
• atoms sharing electrons have different
  attractions for them

Cl has a greater attraction for the electrons
20
Bond Polarity Electronegativity

• Electronegativity
• ability of an atom in a bond to attract the
  shared electrons
• related to electron affinity ionization energy
  but not the same as
• decreases down a group (except for transition
  elements)
• increases across a row
• EN of atom are relative to one another
• range from 0.7 to 4.0

21

• Bond Polarity
• electrons shared between two atoms with different
  EN are shared unequally
• unequal sharing creats a separation of charge --
  polar bond
• greater the DEN of the atoms, the greater the
  polarity of the bond

EN 2.1 EN 3.0
H Cl
partial () charge
partial (-) charge
shared electrons spend more time around Cl
22
Drawing Lewis Structures

• Rules
• write Lewis symbols for each atom in formula
• count total no. of electrons
• count total no. of unpaired electrons divide by 2
  no. of covalent bonds
• arrange atoms (more electropositive in the
  center)
• place correct no. of covalent bonds
• place remaining electrons around atoms so that
  all atoms have octets

23
NH3
8 total electrons 6 upe- 2 3 cov.
bonds
hydrogens are always terminal
24
CO2
16 total electrons 8 upe- 2 4 cov.
bonds
more electropositive atom
25
CO32-


24 total electrons 8 upe- 2 4 cov.
bonds
26
Formal Charge

• Bookkeeping method to keep track of electrons in
  Lewis structures
• Rules
• all unshared electrons assigned to the atom on
  which they reside
• half of all shared electrons are assigned to each
  atom in bond

no. valence e- on free atom
- no. valence e- on bound atom
Formal Charge
27
NH3
N
H
H
H






N 5 e - 5 e - 0
N
H
H
H
28
CO2
C
O
O


29

CO32-
C
O
O
O







-2
0
O
C
0
O
O
30

• Formal charge must equal any charge on the
  structure
• prediction of stability
• most stable structures have lowest sum of
  absolute values of formal charges

0 0 0 0
1 0 1 2
preferred structure